Phosphorescent organic light emitting diodes (OLEDs) are under intensive investigation because of their potential of achieving improved device brightness and performances. In contrast to the fluorescent emission, the electrophosphorescence of heavy transition-metal complexes are easily generated from both singlet and triplet excited states and, thus, the internal quantum efficiency can reach a theoretical level of unity, rather than the 25% inherent upper limit imposed by the formation of singlet excitons for the respective fluorescent counterparts. Thus, a great deal of effort has been spent on the second and third-row transition metal complexes, for developing highly efficient phosphors that can emit all three primary colors.
US 2008-0161568 A1 discloses a phosphorescent tris-chelated transition metal complex comprising i) two identical non-conjugated cyclometalated ligands being incorporated into a coordination sphere thereof with a transition metal, and one ligated chromophore being incorporated into the coordination sphere; or ii) one non-conjugated cyclometalated ligand forming a coordination sphere thereof with a transition metal, and two ligated chromophores being incorporated into the coordination sphere, wherein the metal is iridium, platinum, osmium or ruthenium, and the ligated chromophore possesses a relatively lower energy gap in comparison with that of the non-conjugated cyclometalated ligand, the latter afforded an effective barrier for inhibiting the ligand-to-ligand charge transfer process, so that a subsequent radiative decay from an excited state of these transition complexes will be confined to the ligated chromophore. The architecture and energy gap of the ligated chromophore are suitable for generation of high efficiency blue, green and even red emissions.